diff --git a/src/EventSourcingTests/Bugs/Bug_3661_await_custom_projection_slicing.cs b/src/EventSourcingTests/Bugs/Bug_3661_await_custom_projection_slicing.cs
new file mode 100644
index 0000000000..ceca0afe40
--- /dev/null
+++ b/src/EventSourcingTests/Bugs/Bug_3661_await_custom_projection_slicing.cs
@@ -0,0 +1,75 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using EventSourcingTests.Aggregation;
+using EventSourcingTests.Examples;
+using JasperFx.Core;
+using Marten;
+using Marten.Events;
+using Marten.Events.Aggregation;
+using Marten.Events.Projections;
+using Marten.Internal.Sessions;
+using Marten.Storage;
+using Marten.Testing.Harness;
+using Shouldly;
+using Xunit;
+
+namespace EventSourcingTests.Bugs;
+
+public class Bug_3661_await_custom_projection_slicing : OneOffConfigurationsContext
+{
+    [Fact]
+    public async Task fetching_multiple_items_from_slicers_in_async_custom_projection()
+    {
+        StoreOptions(opts => opts.Projections.Add(new StartAndStopIteratingAwaitablesSlicedProjection(), ProjectionLifecycle.Async));
+
+        var stream = Guid.NewGuid();
+        theSession.Store(new Document1 { Id = stream });
+        theSession.Events.StartStream(stream, new Start(), new Increment(), new Increment());
+
+        var stream2 = Guid.NewGuid();
+        theSession.Store(new Document1 { Id = stream2 });
+        theSession.Events.StartStream(stream2, new Start(), new Increment(), new Increment());
+        await theSession.SaveChangesAsync();
+
+        using var daemon = await theStore.BuildProjectionDaemonAsync();
+        await daemon.StartAllAsync();
+        await daemon.WaitForNonStaleData(20.Seconds());
+
+        var aggregate = await theSession.LoadAsync<StartAndStopAggregate>(stream);
+        aggregate.Count.ShouldBe(2);
+        var aggregate2 = await theSession.LoadAsync<StartAndStopAggregate>(stream2);
+        aggregate2.Count.ShouldBe(2);
+    }
+}
+
+public class StartAndStopIteratingAwaitablesSlicedProjection: CustomProjection<StartAndStopAggregate, Guid>, IEventSlicer<StartAndStopAggregate, Guid>
+{
+    public StartAndStopIteratingAwaitablesSlicedProjection()
+    {
+        UseCustomSlicer(this);
+        IncludeType<Start>();
+        IncludeType<Increment>();
+    }
+
+    public override ValueTask ApplyChangesAsync(DocumentSessionBase session,
+        EventSlice<StartAndStopAggregate, Guid> slice, CancellationToken cancellation,
+        ProjectionLifecycle lifecycle = ProjectionLifecycle.Inline) =>
+        new StartAndStopProjection().ApplyChangesAsync(session, slice, cancellation, lifecycle);
+
+    public ValueTask<IReadOnlyList<EventSlice<StartAndStopAggregate, Guid>>> SliceInlineActions(IQuerySession querySession, IEnumerable<StreamAction> streams) => throw new NotImplementedException();
+
+    public async ValueTask<IReadOnlyList<TenantSliceGroup<StartAndStopAggregate, Guid>>> SliceAsyncEvents(IQuerySession querySession, List<IEvent> events)
+    {
+        var aggregateId = events.First().StreamId;
+        var group = new TenantSliceGroup<StartAndStopAggregate, Guid>(Tenant.ForDatabase(querySession.Database));
+        foreach (var @event in events)
+        {
+            await querySession.LoadAsync<Document1>(@event.StreamId);
+            group.AddEvent(@event.StreamId, @event);
+        }
+        return [group];
+    }
+}
diff --git a/src/Marten/Events/Aggregation/CustomProjection.cs b/src/Marten/Events/Aggregation/CustomProjection.cs
index bff1881c72..d6bab18753 100644
--- a/src/Marten/Events/Aggregation/CustomProjection.cs
+++ b/src/Marten/Events/Aggregation/CustomProjection.cs
@@ -10,7 +10,6 @@
 using Marten.Events.Daemon.Internals;
 using Marten.Events.Projections;
 using Marten.Exceptions;
-using Marten.Internal;
 using Marten.Internal.Sessions;
 using Marten.Internal.Storage;
 using Marten.Schema;
@@ -203,12 +202,12 @@ public virtual bool IsNew(EventSlice<TDoc, TId> slice)
     /// <param name="range"></param>
     /// <param name="cancellation"></param>
     /// <returns></returns>
-    public ValueTask<IReadOnlyList<TenantSliceGroup<TDoc, TId>>> GroupEventRange(DocumentStore store,
+    public async ValueTask<IReadOnlyList<TenantSliceGroup<TDoc, TId>>> GroupEventRange(DocumentStore store,
         IMartenDatabase database,
         EventRange range, CancellationToken cancellation)
     {
-        using var session = store.LightweightSession(SessionOptions.ForDatabase(database));
-        return Slicer.SliceAsyncEvents(session, range.Events);
+        await using var session = store.LightweightSession(SessionOptions.ForDatabase(database));
+        return await Slicer.SliceAsyncEvents(session, range.Events).ConfigureAwait(false);
     }
 
     Type IReadOnlyProjectionData.ProjectionType => GetType();